The combustion of fossil fuels accounts for approximately 80% of the 11,000 mtoe (million tonne oil equivalent) of energy produced world-wide. This results in the production of huge quantities of pollutants, most notably carbon dioxide and other gases that are thought to be the primary contributors to global warming. Concerns about the environmental consequences of fossil fuel combustion are partly responsible for government mandates in the United States and in Europe, which will require manufacturers of passenger vehicles to produce “greener” vehicles that offer better mileage and lower emissions relative to current levels. Manufacturers are working to accomplish this is by offering vehicles that use start-stop technology, and by developing hybrid-electric or all-electric vehicles. Electrical energy storage (EES) devices such as batteries and ultracapacitors are key components for these “green-vehicle” technologies.
Unfortunately, the performance of current EES technologies falls far short of requirements for use in high-efficiency passenger vehicles. For example, batteries have relatively low power density, take a long time to recharge, and have limited cycle-life, and the present ultracapacitors have low energy density. Significant improvements to EES technologies will be required if high-efficiency vehicles are to be deployed broadly.
Batteries employ a Faradaic energy storage mechanism employing a chemical change in oxidation state of the electroactive material via electron transfer at the atomic or molecular level. This mechanism is relatively slow, which limits the power density of the batteries, and further creates stresses that limit cycle-life. Capacitors employ a non-Faradaic mechanism in which energy is stored electrostatically. In this case, there is no change in oxidation state (i.e., no electron transfer at atomic or molecular level). Compared to Faradic processes, non-Faradic processes are very fast, which allows for high power density, and they create little stress in the electroactive materials, which leads to very high cycle-life
Ultracapacitors are EES devices that have the ability to store unusually large amounts of charge compared to comparably-sized capacitors of other types, such as ceramic capacitors, glass capacitors, electrolytic capacitors, etc. For example, an ultracapacitor having the same dimensions as a D-cell battery is capable of storing hundreds of farads (F) of charge. In contrast, an electrolytic capacitor having the same dimensions will typically store a few tens of millifarads (mF) of charge. Thus, ultracapacitors hold promise for storing electrical energy at high power densities, high charge, and high discharge rates for a variety of applications including electric vehicles, hybrid-electric vehicles, industrial equipment, electrical grid load-leveling, and power tools.
While the term ultracapacitor is widely used, those skilled in the art will recognize that term does not have a precise definition and has been used differently by various experts in the field. As used herein, the term ultracapacitor shall refer to an EES device that stores a substantial portion of its charge (greater than about 10%) via formation of electrical double layers at the interface between an electrode coating and a liquid or gel electrolyte. Those skilled in the art will recognize that a variety of names are used for such devices, including electric double-layer capacitors (EDLCs), ultracapacitors, supercapacitors, pseudocapacitors, asymmetric ultracapacitors, hybrid ultracapacitors, battery-ultracapacitor hybrids, and the like. As used herein, the term conventional ultracapacitor shall refer to an EES device that relies predominantly on electrical double layer at two electrodes for charge storage.
Conventional ultracapacitors have energy densities in the range of only from about 1 to about 10 Whr/kg (watt·hours per kilogram) in contrast to secondary cell batteries which have energy densities of from about 10 to 200 Whr/kg. On the other hand, the power density (being a measure of how quickly the energy may be released) for an ultracapacitor is 10 times higher than that of a secondary cell battery or about 1000-5000 W/kg (watts per kilogram).
The high capacitance of a conventional ultracapacitor is obtained by the creation of an electric double layer at the electrode/electrolyte interface in which charges are separated by a distance of a few angstroms. For example, at the anode, negative charge builds on the surface of the electrode, with the electrolyte having a corresponding positive charge at the anode surface. Inversely, at the cathode, positive charge builds on the surface of the electrode, with the electrolyte having a corresponding negative charge. The anode and cathode are separated by a porous separator to give a physical separation to the electrodes, thus preventing short-circuiting, while allowing for the electrolyte to migrate between electrodes and maintain charge balance. The layers of charge at the two electrodes lead to the characterization of conventional ultracapacitors as “electric double-layer capacitors.” The high capacitance of an ultracapacitor arises from the large surface area of the electrode coating coupled with the very small distance (typically several angstroms) between opposing charges in the double layers. In comparison to batteries, ultracapacitors provide higher power density, faster charge-discharge cycles, and longer cycle life. However, ultracapacitors have lower energy density and they tend to be more costly.
FIG. 1 is an illustration of a conventional ultracapacitor 100 having electric double layers. The ultracapacitor 100 includes a cathode 110 and an anode 120 isolated from one another by a porous separator 130. The separator 130 also divides the ultracapacitor cell into a cathode compartment with electrolyte 140 and an anode compartment with electrolyte 150. The electric double layers 160, 170 form at the cathode surface as a cathodic double layer 160 and at the anode surface as an anodic double layer 170. However, as outlined above, such ultracapacitors have limitations.